The present invention relates to forming microwave waveguides for use in level gages to reduce microwave reflection by appropriate waveguide shape transitions, and transitions to barrier materials, together with impedance matching techniques. The techniques are particularly useful in the development of level gages capable of measuring low dielectric constant process fluids and operating at the high frequencies available with present day microwave electronics.
The tuning of microwave components and creation of low reflective waveguide transitions has been attempted for many years by highly skilled engineers and technicians. The techniques that have been used are known by those who work in the field and include pretuned, air filled, off the shelf components that are readily available, as well as transition fittings to perform a number of signal transmission tasks. Some of these components have been integrated into microwave level gages and involve the use of many components to achieve a reasonable amount of tuning. Problems have occurred where there is a transition from a standard rectangular waveguide cross section to a circular cross section, which is desired in cases for non-intrinsically safe sensors that require a mechanical barrier to isolate the sensor, electronics housing, and conduit system from process fluids and vapors. Also the transition from an air filled waveguide to a waveguide filled with barrier materials can degrade performance. Market demands are for higher level measurement accuracy (for example, plus or minus 1 mm) from a physically smaller gage. In order to achieve this performance and the required signal to noise ratio, the tuning and geometry of the waveguide is critical.
Signal reflection in a microwave waveguide that contains a process seal meeting explosion proof/flame proof safety approval requirements has been a source of inefficiencies, losses and false target returns. The present invention reduces losses and inefficiencies to improve microwave level gage accuracy.